1. Field of the Invention
The present invention pertains to data bus systems for transmitting information between a plurality of interconnected and serially arranged terminals, and more particularly, it pertains to data busses of the active type wherein each terminal receives data from one terminal and transmits or retransmits data to the next terminal in the system.
2. Description of the Prior Art
The data bus concept of placing all data onto a single transmission line, or set of transmission lines, and then receiving the data along the line as needed has revolutionized computer architecture and has been instrumental in the development of the microprocessor. This concept is presently being developed for distributing data over larger distances, for example on shipboard, to replace the conventional data distribution systems wherein all receivers and transmitters are directly wired into a central computer or processing unit or wherein a central switchboard functions to distribute messages from and to the various transmitting and receiving units. Such bus systems have the obvious advantage of considerably reducing the amount of cabling required--a factor which can be of considerable importance in the longer distribution systems. Problems are encountered in connection with the longer distribution systems however. For example, at high data rates (i.e., in the megabit range or higher) propagation time between terminals on the bus becomes important and can lead to errors in the received data. Also, with long lines between terminals, the problem of a cable break disabling the entire bus system can become significant.
There are two basic types of data busses which are used in large scale data distribution at the present time, namely, passive busses and active busses. The more typical passive bus system employs transmitters, or drivers, and receivers that couple passively to a transmission line. This is a direct descendant of the current computer data bus technology. The active bus, on the other hand, employs active terminals, each of which receive data from one terminal and then retransmit the data (or transmit new data) to the next terminal in the line with each terminal acting as a potential transmitting and/or receiving point. Thus, in an active data bus, each link between terminals comprises a complete transmission path, whereas in the passive data busses the entire length of the transmission line comprises a single transmission path.
Passive bus systems have a number of disadvantages. For example, a passive transmission line is terminated in its characteristic impedance at each end thereof to absorb the data signal and prevent reflections along the line. The receivers, which tap into the line, must not load the line and will therefore have high impedance inputs. To prevent the transmitters, or drivers, from loading the line, they must be removed from the line when not introducing data which thus has the obvious disadvantage of requiring the rapid switching of power sources into and out of the transmission line.
A variation of the foregoing passive bus system is presently in use wherein all drivers and receivers communicate with the main bus line through separate transmission lines called "stubs". Since each stub presents a reactive discontinuity to the main bus line, even when its driver is "off-line", reflections are introduced at each point that a stub joins the main bus line. These reflections proliferate as the number of stubs increases. Thus, the stubs communicate with the bus line through attenuating "lossy couplers" to reduce the reflections to an acceptable level. While this system works fine at low data rates, when the data rate is high, e.g., in the megabit range or higher, the bit length (defined as the propagating length of a single bit on the data bus line) will oftentimes be less than the bus line length. When this occurs, heroic efforts will be required to prevent the reflections on the line from causing errors in the transmitted data. Furthermore, the large attenuations of the couplers for minimizing the reflections on the line create other problems because of the relatively high driver output level required and the relatively low level of input at the receivers due to the large attenuations. Thus, the receiver stubs tend to be susceptible to the pickup of cross-talk and other forms of electromagnetic interference from nearby machinery, electronic equipment, or power cables. Finally, the receivers operate at a very disadvantageous signal-to-noise ratio at their inputs and therefore require fairly sophisticated hardware in order to extract signals from noise with acceptable accuracy.
Another disadvantage of present data bus systems, particularly the passive bus systems, is the vulnerability of the systems to cable failure whereby a single cable break can disable the entire system even though a portion of the transmitters and receivers on the line are still physically connected. Also, most conventional bus systems have no means of dealing with improper transmission by one of the terminals with the result being--erroneous data on the line.